The invention is based on a priority application EP 06 290 709.2 which is hereby incorporated by reference.
The invention relates to a method of performing a handover of a mobile station in general and to a method of performing a handover of a mobile station by regulating the dataflow to the mobile station during the handover in particular. In other aspects the invention relates to a communication network, to a communication network component and to a computer program product for performing the method in accordance with the invention.
The term handover refers to the process of transferring an ongoing call or data session of a mobile station from a first base station of a communication network to a second base station of the communication network or from one access technology to another access technology. The communication network can for example be a cellular communication network, wherein a first and a second base station serve two adjacent cells. The mobile station can then be placed in a cell that is served by the first base station while having established a connection, e.g. an active call, to the communication network via the first base station. If the mobile station is then physically moved to an adjacent cell served by the second base station then the connection is transferred during the handover phase from the first base station to the second base station. After completion of the handover, the mobile station is connected to the communication network via the second base station.
An active call or data session of a mobile station can also be transferred in a handover process from one access technology to another access technology. For example the mobile station can establish initially a data session by use of a UMTS network which is then handed over to a communication network that employs the IEEE 802.16 (WIMAX) standard.
Within one technology advanced strategies for handover like “soft handover” and “fast base station switch” are possible but such handovers pose additional requirements on the mobile station and restrictions on the air interface. In the generic case the mobile station can only be connected to one base station or one access technology at a time (a change in access technology usually implies also a change of the base station). Thus the mobile station must drop the connection for a brief period of time during handover phase while it is handed over from one base station to the other base station or from one access technology to the other access technology. Such a handover is usually referred to as a hard handover. The connection is broken and then remade. This type of handover is also referred to as a ‘break before make’ type handover.
Since the connection between the mobile station and the communication network is broken temporarily in a break before make type handover there is always a side effect on the services, e.g. on the call or on the data session, that are provided to the mobile station. The requirements to organize this type of handover as seamless as possible vary by the type of protocol a service runs on. On one hand voice over IP (VoIP) and streaming services that rely on real time protocols, e.g. RTP, have little sensitivity for (sparse) packet loss and packet reordering but are severely degraded by delays, jitter and traffic burstiness. On the other hand, services running on the transmission control protocol (TCP) which is used for web browsing, file transfers and downloads cope with delays, jitter and traffic burstiness but react with poor transmission quality on packet loss and especially packet reordering.
According to prior art, a so called bi-cast method is employed during the handover phase. In the bi-cast method, the download dataflow, which is the amount of data that is sent to the mobile station, is duplicated and distributed on each branch in the access serving network (ASN) that is involved in the handover process. For example if the handover is performed between a first and a second base station, then the branches of the access serving node would be the network path from a gateway in the access network to the first base station and to the second base station. The ‘break time’ in a break before make type handover is considerably short because the data traffic is sent to both branches involved in the handover process, but the mobile station cannot be sure to reach the packet flow on the second base station to which it is handed over at the right timing position, because the transmission delay on the different branches may vary. This delay variance is for example caused by a different number of hops that are involved before the data traffic is sent by the base station or by different scheduler strategies or even load on the base stations. So unwanted effects like packet loss, reordering, or duplicate receipts are likely to occur with a random and unpredictable pattern. During handover a considerable amount of blind traffic is produced in the access network by the packet duplication. Thus a considerable amount of resources might be wasted.
Alternatively according to prior art, the dataflow to the mobile station can be totally blocked at the anchor point, which is the point in which the two branches as described above are routed. The total blocking produces the worst case of delay and jitter with the additional risk of buffer overflows at the anchor point.
There is therefore a need for an improved method of performing a handover of a mobile station. There is a need for an improved communication network component, for an improved communication network and for a computer program product that is adapted to perform the method in accordance with the invention.